This invention relates to an array antenna constructed of microstrip elements and, more particularly, to an array antenna wherein each element is formed of a plurality of radiators tuned to radiate in different frequency bands.
Microstrip antenna systems are employed advantageously in spacecraft and other environments requiring a compact antenna structure. An array antenna is constructed readily from a board which is formed of dielectric material and is clad with metallic sheets on opposed surfaces of the board. An array of pad-shaped antenna elements interconnected by electrically conductive metallic strips is etched readily from a metallic sheet on one side of the board. Photolithographic techniques may be employed in the etching to facilitate manufacture and to provide for high precision in the formation of the antenna elements and the interconnecting conductors.
The electrical characteristics of microstrip antenna element are of particular interest in the design of an individual antenna element, as well as in the design of an array of the antenna elements. The thickness of the original board determines the distance between an antenna element on one surface of the board and a ground plane provided by the metallic sheet on the opposite surface of the board. The electrical characteristics are influenced by the distance between element and ground plane. In terms of the electromagnetic operation of a pad-shaped antenna element, the physical structure of the element spaced apart from the ground plane may be likened, for purposes of analysis and understanding of the operation, to an open walled cavity which resonates at specific electromagnetic modes, and with a relatively high value of Q, the ratio of energy stored to energy dissipated per cycle of electromagnetic signal.
As an example in the effect of the distance between element and ground plane upon the electrical characteristics, it is noted that a decreasing of the distance increases the Q of the open-walled cavity, suppresses the development of surface waves which can propagate from element to element along the surface of the array, suppresses blind angles in the viewing of subject matter during a scanning of a beam radiated by the array, and reduces bandwidth to signals which are to be transmitted or received by the array of antenna elements. This dependency of electrical characteristics upon the distance between element and ground plane has necessitated a compromise in the choice of the electrical characteristics for a microstrip array antenna. For example, if the distance has been decreased to avoid surface waves and scan blindness, the resultant antenna may have too narrow a bandwidth to be useful for the performance of a desired mission. The lack of sufficient bandwidth creates a problem in two areas. One area relates to the transmission of a broadband signal, this being a signal having a bandwidth larger than that provided by the foregoing antenna element. The second area of concern relates to the generation of a fan beam which is to be scanned by variation of a frequency of the electromagnetic radiation. By way of example in the generation of such fan beams, one common configuration of an antenna comprises a set of antenna elements, or subarrays which are interconnected by fixed delays. A variation in the frequency of electromagnetic radiation introduces a variation in phase shift among signals outputted by successive ones of the antenna elements or subarrays. A successful scanning of such a fan beam presupposes that each of the antenna elements or subarrays has a sufficiently wide bandwidth to accommodate the shift in frequency. However, in the case of presently available microstrip array antennas, the narrow bandwidth unduly limits the transmission of broadband signals, and the use of a frequency-scanning fan beam.